Electrodeposition of nickel



Patented Sept. l9, 1950 Clifl'ord Struyk, Paterson,

man, Clifl'side Park,

and Stephen C. Doll- J., assignors to Allied Chemical & Dye Corporation, a corporation of New York No Drawing. Application November 28, 1947,

Serial No. 788,685 1 13 Claims. 1

This invention relates to improvements in the electrodeposition of nickel.

Attempts have been made to prevent pitting of nickel deposits obtained by electrodeposition thereof from nickel electroplating baths by incorporation of various types of compounds into such baths. However, in certain instances use of such compounds have proven unsatisfactory in practice. This is true particularly in the case of nickel fluoborate electroplating olutions, owing to poor solubility and/or stability of such adjuvants under electrolysis conditions.

One object of the invention is to obtain satisfactory nickeldeposits by electrodeposition. Another object is to provide improved nickel electroplating baths and methods for electrodeposition from such baths of nickel in the form of pit-free uniform deposits. Yet another object is the provision of improved nickel electroplating bathssuch as those of nickel sulfate, nickel chloride and particularly nickel fluoborate-containing certain agents which have been found to prevent pitting of the nickel electrodeposits.

Other objects and advantages will in part be obvious and in part appear hereinafter.

In accordance with the present invention it has been found that the objectives thereof may be accomplished in the electrodeposition of nickel by incorporating in an aqueous nickel salt solution, an ester of a sulfonated aliphatic tricarboxylic acid which ester contains from ,8 to 25 carbon atoms. More particularly, it has been found that the presence of a small amount of such sulfonated aliphatic tricarboxylic acid ester, preferably in the form of a salt of the ester, in an aqueous electroplating solution of a nickel salt such as nickel sulfate, nickel chloride or especially nickel fluoborate eliminates pitting and formation of pin holes, and results in brighter, more uniform deposits of nickel.

The sulfonated aliphatic tricarboxylic acid esters utilized in accordance with the present invention correspond with the general formulacoox R-COOX coox -wherein R is an aliphatic radical containing at least one sulfo group in the form of the free sulfonic acid or a salt thereof and X'is a member of the group consisting of hydrogen and a radical obtained by dehydroxylation of an alcohol, not

more than two of said X substituents being hydrogen, said esters containing from 8 to 25 carbon atoms.

The aliphatic tricarboxylic acids which may be employed in making the esters utilized in accordance with the present invention may be symmetrical or unsymmetrical, saturated or unsaturated tricarboxylic acids, as for example ethane tricarboxylic acid, propane a, a, y-tricarboxylic acid, tricarballyli-c acid, propane a, p, B-tricarboxylic acid, propane a, a, ,e-tricarboxylic acid, a-carboxy adipic acid, isobutane tricarboxylic acid and the unsaturated acids ethylene tricarboxylic acid, iso aconitic acid, aconitic acid, 'y-butylene a, B, 'y-tricarboxylic acid and methyl aconitic acid. Although the above acids are typical of those which may be employed in accordance with our invention, it is apparent that other acids of the type falling within the above general formula may be used. The preferred acid for the purposes of this invention is aconitic acid.

A large variety of alcohols may be used for the formation of the esters of the present invention. Generally, any of the monohydric paraflin alcohols such as methyl and ethyl alcohol as well as the various isomers of propyl, butyl, amyl, hexyl, heptyl, octyl and higher alcohols as for example dodecyl, hexadecyl, cetyl, oleyl and stearyl may be employed. Instead of using the pure alcohols various commercial alcohols or mixtures of a100- hols may be utilized such as the mixture of higher alcohols formed as a by-product in the manufacture of methanol from carbon monoxide and hydrogen. Also various polyhydric and other alcohols may be applied such as ethylene glycol, butylene glycol, diethylene glycol, and the methyl, ethyl, propyl etc. ether of ethylene or diethylene glycol or their homologues. Typical cyclic alcohols which are also of use in the present invention include cyclohexanol, methyl cyclohexanol, benzyl alcohol, furfufyl alcohol, -terpineol, borneol, phenol, xylenols, parabutyl phenols and other alkylated phenols or their hydrogenation products. We have found that the esters of sulfonated aconitic acid with an aliphatic alcohol such as butyl or amyl alcohol are very suitable and convenient to use in the nickel electroplating baths of the invention and give good results.

Although we prefer to employ the tri-esters in practice, the diesters and monoesters of the above noted aliphatic tricarboxylic acids are also suitable, provided the total number of carbon atoms present in such esters ranges from 8 to 25. Further, various mixed esters may be applied. A tricarboxylic acid may be esterified with any combination of alcohols including the monohydric saturated or unsaturated alcohols, aliphatic polyhydric alcohols, their esters having at least one remaining free hydroxyl group and other P hydric alcohols containing at least one free hydroxyl group and in which one or more hydroxyl groups have'been previously reacted with a suitable acid. Also included are acid alcohols such as ricinoleic acid containing one or more free bydroxyl groups and in the remainders of which carboxylic acid residues have been introduced.-

The tricarboxylic acid esters of the invention may be prepared by reaction of any of the above tricarboxylic acids with any of the above described alcohols by way of the conventional methods of esterification.

One or more sulfo groups are attached to the aliphatic portion (designated by R in the general formula) of the tricarboxylic acid esters employed herein. Incorporation of such sulfo groups into the compounds of the invention may be accomplished by various sulfonation procedures. Thus, for example, saturated tricarboxylic acids may be sulfonated with strong sulfonating agents such as sulfur trioxide or fuming sulfuric acid to obtain the corresponding tricarboxylic acids containing sulfo groups, or halogen substituted saturated or unsaturated tricarcarboxylic acids may be treated with an alkali metal sulfite whereby a sulfo group is substituted for a halogen atom. Most desirably, however, the sulfonated aliphatic tricarboxylic acid is prepared from the corresponding unsaturated acid by addition reactions with salts of sulfurous acid. Thus, aconitic acid reacts with ammonium or alkali metal bisulfites by way of addition to form sulfonated aconitic acid which is a saturated compound also known as sulfo tricarballylic acid. The sulfonated tricarboxylic acids obtained in the above manner may then be esterified. Alternatively, the above noted sulfonation procedures may be carried out after esterification of the tricarboxylic acids. Accordingly, throughout the specification andclaims the terms "ester of a sulfonated aliphatic tricarboxylic acid" and sulfonated aliphatic tricarboxylic acid ester are intended to denote and include tricarboxylic acid esters which have been prepared by sulfonation either before or after esterification of the tricarboxylic acid.

The preferred compounds utilized in accordance with the invention are the salts of the esters of sulfonated aliphatic tricarboxylic acids. Throughout the specification and claims the expression a salt of an ester is intended to define esters which contain a basic salt-forming radical linked to a free sulfo group of the tricarboxylic acid ester. The salt-forming radicals which may be attached to the free sulfo groups include ammonium, an alkali metal, e. g. sodium and potassium, and other metals such as nickel itself, which is the metal plated from the bath, and the radical of an organic base, e. g. trimethyl amine and pyridine. Formation of the above salts of the sulfonated tricarboxylic acid esters of the invention is desirable for the purpose of enhancing the solubility of such esters.

The foregoing salt-forming radicals may be incorporated into one or more of the free sulfo groups of the tricarboxylic acid esters of the invention in any suitable manner, e. g. by neutralizing such free sulfo groups with the appropriate base to form the corresponding sulfo salt of the ester. However, in the case of unsaturated tricarboxylic acid esters, sulfonation of which may be carried out by addition reactions with ammonium or alkali metal bisulfites, as indicated above, a suli'o salt may be formed directly as a result of sulfonation in this manner, e. g. sulfonation of an aconitic acid ester by means of sodium bisulfite results in formation of the sodi- 0f the above described sulfonated tricarboxylic acid esters utilized in accordance with the invention, the preferred esters are those which may be represented by the following general formula- COOK YOSOzR-COOX COOX ' wherein R is an aliphatic radical, Y is a member of the group consisting of hydrogen, ammonium, the radical of an organic base, and a metal, and X is a member of the group consisting of hydrogen and a radical obtained by dehydroxylation of an aliphatic alcohol, not more than two of said X substituents being hydrogen, said esters containing from 8 to 25 carbon atoms. Specific illustrations of some of the values of R, Y and K have been mentioned above.

The following examples serve to illustrate methods of preparing the sulfonated aliphatic tricarboxylic acid esters and salts of such esters suitable for use in the invention:

Example 1.-Ethane tricarboxylic acid is dissolved in fuming sulfuric acid and the reaction is allowed to continue for a period of time under controlled temperature conditions. Excess sulfuric acid is then neutralized with barium hydroxide and the remaining solution is found to contain a mixture of mono sulfo and disulfo ethane tricarboxylic acids. This mixture is then esterified with 2-ethyl butanol in known manner to form a mixture of the corresponding esters of such acids.

Example 2.Dry hydrochloric acid gas is passed into a mixture of citric acid and butyl alcohol under conditions known in the art to obtain as a reaction product tributyl citrate. The citric acid ester is then reacted with acetyl chloride in known manner to dehydrate the citric acid residue and form tributyl aconitate which latter compound is sulfonated by addition of sodium bisulfite thereto in the conventional way to produce as a reaction product a saturated compound which is the sodium salt of sulfonated tributyl aconitate, also known as tributyl sodium sulfo-tricarballylate.

Example 3.Aconitic acid is reacted with sodium bisulfite under known addition reaction procedure to produce as the addition compound the saturated sodium salt of sulfonated aconitic acid, alternatively known as the sodium salt of sulfo-tricarballylic acid. The latter compound is esterifieddn known manner by treatment with amyl acohol to produce the sodium salt of sulfonated triamyl aconitate.

In order to be suitable for the purposes of the invention it is required that the above sulfonated aliphatic tricarboxylic acid esters and salts of such esters contain from 8 to 25 carbon atoms. We have found that those esters and salts of such esters which contain more than 25 carbon atoms are too insoluble to be operable in the nickel electroplating solutions of the invention. It the number of carbon atoms in the above compounds is less than 8, such compounds have been found to have little eilect as assistants in preventing pitting of electrodeposited nickel in accordance with the teachings of the invention.

Only a very small amount of the sulfonated aliphatic tricarboxylic acid anti-pitting agents of the invention is necessary to obtain nickel deposits which are uniform and pit-free. The amount of ester, salt of such ester or mixture of both which may be incorporated into nickel plating solutions may vary from as little as .01 gram per liter to saturation. No advantage is obtained in employing quantities of the tricarboxylic acid esters of the invention in excess of the amount which will dissolve in the particular nickel electroplating solution utilized. In usual commercial operation when employing the preferred antipitting agents of the invention, namely. the alkali metal, preferably sodium salt, of sulfonated tributyl or triamyl aconitate, the amount of the more soluble butyl ester utilized may range from 0.1 to 2 grams per liter of solution, best results being obtained when operating in the neighborhood of about 1 gram per liter, while the amount of the amyl ester applied may range from .03 to .20 gram per liter of solution with best results realized when operating in the neighborhood of about 0.1 gram per liter. Of the above two compounds preferred for use in the present invention, we have found the application of the butyl ester to be more desirable.

The nickel plating baths contemplated for use in the present invention may contain various other adjuvants in addition to the particular nickel salt employed as an electrolyte and the small amount ofsulfonated tricarboxylic acid ester incorporated therewith. Thus, free boric acid in moderate amounts and small amounts of nickel chloride may be added to the nickel fluoborate baths which we preferably employ. In the nickel sulfate and nickel chloride baths containing the tricarboxylic acid esters of the invention free boric acid may also be included. The various adjuvants noted directly above aid to bring about formation of a strong uniform nickel deposit under the most favorable operating conditions including a short electrolysis period. Thus, for example, boric acid acts as a buiier to maintain proper acidity of solution while the nickel chloride is employed primarily to effect rapid anode corrosion.

In order to realize the optimum results under commercial operating conditions the nickel electroplating solutions of the present invention are electrolyzed at comparatively high temperatures, for example above 100 F., thus increasing the conductivity of the solution and thereby increasing the current density and efliciency at which the electroplating operation may be carried out. The ability to employ high current densities cuts down the operating time required to deposit a given thickness of nickel. To obtain good anode corrosion and high efilciency it is also desirable to maintain the plating solution sufliciently acid at all times, which condition may be satisfied by having present proper amounts of acidic material such as sulfuric or fiuoboric acid, the latter being particularly preferred when employing nickel fiuoborate solutions. The additional use of a buil'ering agent such as the above noted boric acid is often advantageous, especially in connection with nickel fluoborate solutions.

While the foregoing described principles of the invention are applicable to the electrodeposition of nickel from any aqueous nickel salt solution, the present improvements are particularly directed to utilization of baths comprising aqueous solutions oi nickel sulfate, nickel chloride and nickel fluoborate. The preferred embodiment of the invention is directed to utilization of nickel fluoborate baths.

The overall and preferred ranges oi the various ingredients and operating conditions for a nickel fluoborate bath are noted below:

Table I Ranges 0! Amounts Employed in Grams/Liter Ingredients Overall Preferred Nickel fluoborate Ni (BF $4500 280-31) Free fluoborlc acid Him. 4-40 8-35 Free boric acid H1803. 3.5-40 10-35 Nickel chloride NiCl2.6H2O 0-30 0-10 Sodium salt of sulionuted tributyl .0l1iaturation 0.1-2.0

aconitate, or Sodium salt of sulfonated triamyl .01-saturation .03-.20

aconltate.

OPERATING CONDITIONS Temperature, F 65-200 -170 pH (colorimetric) 1.0-4.0 2. H. 5 Oath??? current density (Amps. per l0-1,000 60-250 sq. Anode-cathode area ratios 0. 75-2 l-l Voltage 1-14 2-10 The preferred ranges of the various ingredients and operating conditions for a nickel sulfate (Watts type bath) are given in the following table:

Table II The preferred ranges of the various ingredients and operating conditions for a nickel chloride bath are listed below:

Table III Ingredients Grams/Liter Nickel chloride NiClz.6H:O 280-320 Boric acid H3130: 25-35 Sodium salt of sulfonated tributyl aconitate, or. 0. l-2. 0 Sodium salt of sulfonated triamyl aconitate 03-. 20

OPERATING CONDITIONS Temperature, F -190 nH (colorimetric) 1.0-4.0 Cathode current density 10-200 Anode-cathode area ratio l-l Voltage 1-6 7 The following examples illustrate practice of the present invention:

Example 4.--Nickel fluoborate bath.

' Grams/liter Nickel fluoborate Ni (BEE): 300 Free fluoboric acid HBF4 14 Free boric acid HaBOa 30 Nickel chloride NlClz.6HzO 1.0

Sodium salt of sulfonated tributyl aconitate 1.0

Operating conditions:

Temperature, F 130 pH (colorimetric) 2-3.5 Cathode current density (amps per sq. ft.) 50-250 Voltage 1-8 Example 5.-.06 gram per liter of the sodium salt of sulfonated triamyl aconitate was substituted for the sulfonated tributyl aconitate of Example 4.

Example 6.-Nickel sulfate (Watts type) bath.

v Grams/liter Nickel sulfate NiSOflHzO 240 Nickel chloride NiCl2.6HzO 45 Boric acid H3303 30 Sodium salt of sulfonated tributyl aconitate 1.0 Operating conditions:

Temperature, "F 130 pH (electrometric) 4-5.5 Cathode current density (amps per sq. ft.) 5-75 Voltage 1-4 Example 7 .0.1 gram of the sodium salt of sulfonated triamyl aconitate was substituted for the sulfonated tributyl aconitate of Example 6.

Example 8.-'-Nickel chloride bath.

Grams/liter Nickel chloride NiC12.6H2O 300 Boric acid H3303 I 30 Sodium salt of sulfonated tributyl aconitate 1.0

Operating conditions:

Temperature, "F 140 pH (colorimetric) 1.5 Cathode current density (amps per sq. ft.) I Voltage 2 plating baths agitation of such baths by air or by mechanical means may. or may not be prac-. ticed as desired. The use of agitation generally increases the permissible current density at which sound deposits can be obtained and thus enables production of higher plating rates. Moreover,

especially when operating at high temperatures,-

some means of agitation is preferably employed to avoid local overheating. All commercial types of anodes may be utilized with the nickel electroplating baths described and claimed herein.

The tricarboxylic acid "esters and salts ,there-' of as above defined are especially efiective in.

nickel fluoborate electroplating solutions for pre-. vention of pitting of electrodeposited nickel therefrom because of the good solubility and excellent stability of such organic compounds in nickel fluoborate solutions under rigorous electrolysis conditions. We have found that the tricarboxylic acid esters of the invention are of particular value in the more commercially useful concentrated nickel fiuoborate solutions containing from 250-450 grams per liter of nickel fluoborate wherein prior art anti-pitting agents .are notably defective from standpoint of poor solubility and/or stability. Hence, in the practice of the invention, use of nickel fluoborate solutions having concentrations of not less than 250 grams per liter are preferred. Further, the tricarboxylic acid compounds of the invention are substantially non-foaming in the nickel electroplating baths in which they are incorporated and thus permit agitation of such baths either by air or mechanical means, whereas this advantageous expedient is not permissible in the case of the application of other anti-pitting compounds which cause foaming.

While the invention has been described particularly with respect to the use of the above suli'onated tricarboxylic acid esters and salts of such esters separately in nickel electroplating solutions, such compounds may also be utilized in various admixtures with each other to produce the efiect desired.

It will be realized by those skilled in the art that changes may be made in the nickel electroplating process and in the composition of the nickel electroplating baths of the invention without departing from the spirit thereof. The invention is, therefore, to be taken as limited only by the scope of the appended claims.

Reference is made to our co-pending application, Serial No. 788,684, filed November 28, 1947, which claims related subject matter.

We claim:

1. In the process of electrodepositing nickel, the improvement which comprises electrolyzing an aqueous acid solution containing a nickel salt of the group consisting of nickel sulfate, nickel chloride and nickel fluoborate, and from .01 gram per liter up to saturation of a salt of a sulfonated ester of aconitic acid and an aliphatic alcohol, said salt of the ester containing from 8 to 25 carbon atoms.

2. In the process of electrodepositing nickel the improvement which comprises electrolyzing an aqueous acid nickel fluoborate solution containing 0.1 to 2 grams per liter of the sodium salt of sulfonated tributyl aconitate.

3. In the processof electrodepositing. nickel the improvement which comprises electrolyzing an aqueous acid nickel fiuoborate solution containing .03 to .20 gram per liter of the sodium salt of sulfonated triamyl aconitate.

4. An electroplating bath comprising an aqueous acid solution of a nickel salt of the group consisting of nickel sulfate, nickel chloride and nickel fluoborate, and 0.1 to 2 grams per liter of the sodium salt of sulfonated tributyl aconitate.

5. An electroplating bath comprising an aqueous acid solution of a nickel salt of the group consisting of nickel sulfate, nickel chloride and nickel fiuoborate, and .03 to .20 gram per liter of the sodium salt of sulfonated triamyl aconitate.

6. An electroplating bath comprising an aqueous acid solution containing not less than 250 grams per liter of nickel fluoborate and from .01 gram per liter up to saturation of a salt of a sulfonated ester of aconitic acid and an aliphatic alcohol, said salt of the ester containing from 8 to 25 carbon atoms.

7. In the process of electrodepositing nickel, the improvement which comprises electrolyzing an aqueous acid nickel salt solution containing a minor proportion of an ester of sulfonated aconitic acid and an aliphatic alcohol, said ester containing from 8 to 25 carbon atoms.

8. The process as defined in claim 1 wherein the nickel salt is nickel fluoborate.

9 The process as defined in claim 1 wherein the nickel salt is nickel sulfate.

10. The process as defined in claim 1 wherein the nickel salt is nickel chloride.

11. An electroplating bath comprising an aqueous acid solution of a nickel salt of the group consisting of nickel sulfate, nickel chloride and nickel fluoborate and from .01 gram per liter up to saturation of a salt of a sulfonated ester of aconitic acid and an aliphatic alcohol, said salt of the ester containing from 8 to 25 carbon atoms.

REFERENCES CITED The following references are of record in the file 01 this patent:

UNITED STATES PATENTS Number Name Date 2,389,135 Brown Nov. 20, 1945 2,389,180 Brown Nov. 20, 1945 2,466,677 Brown Apr. 12, 1949 OTHER REFERENCES Transactions of the Kansas Academy of Science, vol. 48 (1945) pages 173, 174. 

1. IN THE PROCESS OF ELECTRODEPOSITING NICKEL, THE IMPROVEMENT WHICH COMPRISES ELECTROLYZING AN AQUEOUS ACID SOLUTION CONTAINING A NICKEL SALT OF THE GROUP CONSISTING OF NICKEL SULFATE, NICKEL CHLORIDE AND NICKEL FLUOBORATE, AND FROM .01 GRAM PER LITER UP TO SATURATION OF A SALT OF A SULFONATED ESTER OF ACONITIC ACID AND AN ALIPHATIC ALCOHOL, SAID SALT OF THE ESTER CONTAINING FROM 8 TO 25 CARBON ATOMS. 